1. Field of the Invention
The present invention relates to an ink jet printing apparatus that carries out printing using a print head based on an ink jet method, as an information output apparatus for outputting information from a personal computer or the like or in an image forming apparatus such as a copier or a facsimile machine.
2. Description of the Related Art
Ink jet printing apparatuses form an image by ejecting ink from a plurality of printing elements provided in a print head so that the ink impacts a print medium. The ink jet printing apparatus ejects a large number of small ink droplets the amount of which is small. Accordingly, depending on the contents of the ink or the usage of the print head, thermal decomposition may occur in a dye or pigment contained in the ink. As a result, immobilized ink may be deposited near ejection openings. If no action is taken for the deposition of such solids, the openings may be gradually blocked by the solids to disturb the direction or amount of ejections. Moreover, the printing element itself may become unable to eject ink. Images formed by the print head which has become unable to eject ink correctly undergo a non-uniform density or the presence of stripes. Such images thus have low quality.
Thus, the ink jet printing apparatus contains a recovery mechanism used for the maintenance of the print head. The recovery mechanism executes various maintenance processes. Common such processes include suction recovery that forcedly sucks a predetermined ink from the printing element in the print head, preliminary ejection that causes ejection from the printing element in the print head which ejection is not related to printing, and a wiping operation of using a wiper blade to clean an ejection surface in which the ejection openings are arranged. These processes are executed at proper points to keep the print head and outputted images reliable.
One of the maintenance processes is preliminary ejection carried out immediately before printing. In an unused printing element, an ink solvent is volatilized and a color material is correspondingly concentrated. This phenomenon becomes gradually serious over time. Thus, in order to efficiently reduce this phenomenon while minimizing the loss of time and ink, it is appropriate to carry out preliminary ejection immediately before printing to forcedly dispose of the concentrated ink. In general, this process is started at the same time when a command to start printing is given and is executed concurrently with an operation of feeding a print medium. This prevents the time required for the maintenance of the print head from affecting the time required for printing (that is, throughput).
However, for the recent ink jet printing apparatus, there have been growing demands for smaller sizes in view of space saving. Furthermore, arrangements focusing on portability have been desired. If attempts are made to achieve this, the following new problems arise.
A size reduction reduces the distance over which feeding means feeds one print medium from a position where print media are stacked into the printing apparatus main body after separating this medium from the others. Accordingly, a paper feeding operation can be completed in a shorter time. Printing throughput is thus expected to be further improved.
However, if a paper feeding operation and a preliminary ejection operation are performed concurrently, the throughput is not improved if only the time required for the paper feeding operation is reduced while the preliminary ejection operation requires a time longer than that required for the paper feeding operation. The throughput is limited by the time required for the preliminary ejection operation, which is not related directly to printing. In the prior art, a preliminary ejection sequence executed before printing is advantageous in that it ensures reliability without affecting the throughput in a series of printing operations from paper feeding to paper discharging. However, a smaller printing apparatus with a reduced paper feeding time does not give such an advantage based on the preliminary ejection sequence but conversely hinders the improvement of the throughput.
On the other hand, if focus is on a size reduction or portability, the capacity of a power source that can be mounted in the printing apparatus is also limited. If the capacity of the power source is reduced, it is difficult to simultaneously drive a number of driving sources. For example, the following motors are often driven at substantially the same time when printing is started: a carriage motor used to drive a carriage on which the print head is mounted and which moves for scanning, a conveying motor used to drive conveying means for conveying a print medium, and a feeding motor used to drive the feeding means for feeding a print medium to the conveying means. However, if the capacity of the power source is limited, these three motors may not be driven at exactly the same time. To cope with this, it is possible to execute a series of sequences while sequentially switching the driven motor at intervals of a short time. However, this method further reduces the throughput.